There are various commercial and industrial uses for high pressure fluid pump systems operating at pressures greater than 20,000 psi. Such pump systems can be used in, for example, fluid-jet cutting systems, fluid-jet cleaning systems, etc. Fluid-jet cutting systems often use reciprocating, positive displacement pumps (e.g., crankshaft-driven plunger pumps). Crankshaft-driven plunger pumps, such as triplex plunger pumps (i.e., pumps having three cylinders and associated plungers) operating at outlet pressures of 20,000 psi or more produce pressure pulsations caused by the cyclic output from the pump cylinders. These pressure pulsations can produce undesirably high levels of pressure ripple downstream from the pump. The pressure ripple can be partially mitigated by use of a pump output manifold that contains a volume of the high pressure fluid before it flows to downstream applications.
Conventional low pressure crankshaft-driven, reciprocating positive displacement pumps operating at outlet pressures of 7,500 psi or less typically use pistons instead of plungers. One reason for this is that piston pumps generally have much higher volumetric efficiencies that plunger pumps. Piston pumps, however, can also create significant pressure pulsation during operation. As a result, such pumps are typically used with pulsation dampeners to reduce pressure ripple downstream of the pump. Pulsation dampeners typically include a vessel having a resilient diaphragm with a gas (such as nitrogen) on one side of the diaphragm and the media being pumped (e.g., water) on the opposite side of the diaphragm. In operation, water discharged from the pump flows into the dampener vessel, with the diaphragm alternatingly expanding and compressing the gas as the water pressure increases, and then contracting and letting the gas expand against the water as the water flows out of the vessel and the pressure decreases. Pulsation dampeners are usually attached directly to the output manifold of the pump. In this way, dampeners can reduce pressure pulsations in the water downstream from the pump.
Gas filled pulsation dampeners tend to lose effectiveness as output pressures increase and the gas begins to go through a phase change to a liquid or supercritical fluid. As noted above, high pressure pumps typically rely primarily on the volume of fluid in the output manifold to reduce pressure ripple. Pressure attenuators can also be used to mitigate pump pressure ripple. Pressure attenuators are essentially pressure vessels that accumulate the high pressure water from the pump cylinders to dampen pressure fluctuations in the water as it is provided to, for example, a fluid-jet cutting head or other downstream application. Pressure attenuators are generally placed as close to the pump as possible, but even with relatively large attenuators, these systems can still experience relatively large pressure fluctuations during pump operation that results in downstream pressure ripple.
Fluid-jet systems (e.g., waterjet or abrasive-jet systems) are one of the areas of technology that utilize ultrahigh pressure pumps. Fluid-jet systems can be used in precision cutting, shaping, carving, reaming, and other material-processing applications. The liquid most frequently used to form the jet is water, and the high-velocity jet may be referred to as a “water jet” or “waterjet.” In operation, waterjet systems typically direct a high-velocity jet of water toward a workpiece to rapidly erode portions of the workpiece. Abrasive material can be added to the fluid to increase the rate of erosion. When compared to other shape-cutting systems (e.g., electric discharge machining (EDM), laser cutting, plasma cutting, etc.), waterjet systems can have significant advantages. For example, waterjet systems often produce relatively fine and clean cuts, typically without heat-affected zones around the cuts. Waterjet systems also tend to be highly versatile with respect to the material type of the workpiece. The range of materials that can be processed using waterjet systems includes very soft materials (e.g., rubber, foam, leather, and paper) as well as very hard materials (e.g., stone, ceramic, and hardened metal). Furthermore, in many cases, waterjet systems are capable of executing demanding material-processing operations while generating little or no dust, smoke, and/or other potentially toxic byproducts.
In a typical waterjet system, a pump pressurizes water to a high pressure (e.g., up to 60,000 psi or more), and the water is routed from the pump to a cutting head that includes an orifice. Passing the water through the orifice converts the static pressure of the water into kinetic energy, which causes the water to exit the cutting head as a jet at high velocity (e.g., up to 2,500 feet per second or more) and impact a workpiece. In many cases, a jig supports the workpiece. The jig, the cutting head, or both can be movable under computer and/or robotic control such that complex processing instructions can be executed automatically.
The pressure ripple produced by conventional crankshaft-driven plunger pumps used in waterjet systems have a number of disadvantages. For example, the pulsations can cause vibration and fatigue in the fluid conduits and other components that make up the high pressure fluid circuit between the pump and the cutting head. Additionally, the pressure pulses can cause vibration of the cutting head, which adversely affects the waterjet cutting quality. As discussed above, methods for mitigating pressure ripple typically include increasing the volume of the pump manifold or adding a pressure attenuator to the system. Although somewhat effective, neither approach is an ideal solution. Pressure manifolds typically have cross-bores that receive the output flow from each pump cylinder. The cross-bores within the manifold can create areas of high stress concentrations that limit component life due to eventual fatigue failure. In addition, pressure manifolds can be relatively expensive to manufacture, and the cost generally increases as the size of the manifold increases. As noted, some pumps are fitted with pressure attenuators to reduce pressure ripple and mitigate the disadvantages discussed above. As with pressure manifolds, however, large pressure attenuators can also be costly to manufacture due to component size. Although attenuators do not have cross-bores, they are also subject to fatigue failure. In addition, increasing the volume of pressurized water stored in a pressure manifold or attenuator has the downside of increasing stored energy within the pump system. Moreover, neither output manifolds nor pressure attenuators provide the full extent of pulse attenuation desired. Accordingly, it would be desirable to have waterjet pump systems that produce less pressure ripple than conventional pump systems to reduce fatigue failures and enhance cutting quality.